Functional group migration in benzaldoxime- O- n-propyl ether radical cation

Abstract

The mechanism of the elimination of CH 2O and C 2H 4O from the molecular ions of benzaldoxime- O- n-propyl ether ( 1) was studied using specifically deuterated derivatives, methods of tandem mass spectrometry, and ab initio calculation of the minimum energy reaction path (MERP) at the B3LYP/6-311G ∗//UHF/3-21G ∗ level of theory. The results show that the eliminated CH 2O (major) and C 2H 4O (minor) contain specifically the α- and β-CH 2 groups, respectively, of the propoxy chain of 1, requiring a rearrangement of the molecular ion of 1 by a 1,5-shift of the benzaldimine moiety along the propoxy chain. This rearrangement reaction follows the general Longevialle mechanism of functional group transposition along an aliphatic chain by rearrangement of distonic ions. In the case of 1 + the initial δ-distonic ion 1dist is generated by a 1,5-H shift from the terminal CH 3 group to the N-atom of the oxime group. The compound 1dist is converted to the α-distonic isomer 5dist by cyclization of the molecular ion of N-benzyl-1,2-oxazolidine ( 5) and by subsequent ring opening into the δ-distonic isomer 2dist of N-(3-hydroxypropyl)benzaldimine ( 2) (see Scheme 4). Eventually the distonic ion 2dist fragments by loss of CH 2O and C 2H 4O. The possibility that the distonic isomers of 1 + and 2 + interconvert with their cyclic isomers 2H-3,4,5,6-tetrahydro-3-phenyl-1,2-oxazine radical cation ( 3 +) and 2H-3,4,5,6-tetrahydro-3-phenyl-1,3-oxazine radical cation ( 4 +) was also examined by comparing the EI- and MIKE-spectra and the CID of the molecular ions of the isomers and by ab initio calculation.